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Unit 3 – Stereochemistry . Stereoisomers Chirality (R) and (S) Nomenclature Depicting Asymmetric Carbons Diastereomers Fischer Projections Stereochemical Relationships Optical Activity Resolution of Enantiomers. Stereochemistry. Stereochemistry:

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Unit 3 stereochemistry l.jpg

Unit 3 – Stereochemistry



(R) and (S) Nomenclature

Depicting Asymmetric Carbons


Fischer Projections

Stereochemical Relationships

Optical Activity

Resolution of Enantiomers

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  • Stereochemistry:

    • The study of the three-dimensional structure of molecules

  • Structural (constitutional) isomers:

    • same molecular formula but different bonding sequence

  • Stereoisomers:

    • same molecular formula, same bonding sequence, different spatial orientation

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  • Stereochemistry plays an important role in determining the properties and reactions of organic compounds:

Caraway seed


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  • The properties of many drugs depends on their stereochemistry:




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  • Enzymes are capable of distinguishing between stereoisomers:

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Types of Stereoisomers

  • Two types of stereoisomers:

    • enantiomers

      • two compounds that are nonsuperimposable mirror images of each other

    • diastereomers

      • Two stereoisomers that are not mirror images of each other

      • Geometric isomers (cis-trans isomers) are one type of diastereomer.

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  • Enantiomers are chiral:

    • Chiral:

      • Not superimposable on its mirror image

  • Many natural and man-made objects are chiral:

    • hands

    • scissors

    • screws (left-handed vs. right-handed threads)

Right hand threads slope up to the right.

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  • Some molecules are chiral:

Asymmetric (chiral) carbon

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Asymmetric Carbons

  • The most common feature that leads to chirality in organic compounds is the presence of an asymmetric (or chiral) carbon atom.

    • A carbon atom that is bonded to four different groups

  • In general:

    • no asymmetric C usually achiral

    • 1 asymmetric C always chiral

    • > 2 asymmetric C may or may not be


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Asymmetric Carbons

Example: Identify all asymmetric carbons present in the following compounds.

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  • Many molecules and objects are achiral:

    • identical to its mirror image

    • not chiral

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Internal Plane of Symmetry

  • Cis-1,2-dichlorocyclopentane contains two asymmetric carbons but is achiral.

    • contains an internal mirror plane of symmetry

  • Any molecule that has an internal mirror plane of symmetry is achiral even if it contains asymmetric carbon atoms.


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Internal Plane of Symmetry

  • Cis-1,2-dichlorocyclopentane is a meso compound:

    • an achiral compound that contains chiral centers

    • often contains an internal mirror plane of symmetry

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Internal Plane of Symmetry

Example: Which of the following compounds contain an internal mirror plane of symmetry?

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Chiral vs. Achiral

  • To determine if a compound is chiral:

    • 0 asymmetric carbons: Usually achiral

    • 1 asymmetric carbon: Always chiral

    • 2 asymmetric carbons: Chiral or achiral

      • Does the compound have an internal plane of symmetry?

        • Yes: achiral

        • No:

          • If mirror image is non-superimposable, then it’s chiral.

          • If mirror image is superimposable, then it’s achiral.

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Conformationally Mobile Systems

  • Alkanes and cycloalkanes are conformationally mobile.

    • rapidly converting from one conformation to another

  • In order to determine whether a cycloalkane is chiral, draw its most symmetrical conformation (a flat ring).

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Chiral vs. Achiral

Example: Identify the following molecules as chiral or achiral.



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(R) And (S) Nomenclature

  • Stereoisomers are different compounds and often have different properties.

  • Each stereoisomer must have a unique name.

  • The Cahn-Ingold-Prelog convention is used to identify the configuration of each asymmetric carbon atom present in a stereoisomer.

    • (R) and (S) configuration

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(R) and (S) Nomenclature

  • The two enantiomers of alanine are:

Natural alanine

Unnatural alanine



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(R) and (S) Nomenclature

  • Assign a numerical priority to each group bonded to the asymmetric carbon:

    • group 1 = highest priority

    • group 4 = lowest priority

  • Rules for assigning priorities:

    • Compare the first atom in each group (i.e. the atom directly bonded to the asymmetric carbon)

      • Atoms with higher atomic numbers have higher priority

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(R) and (S) Nomenclature









Example priorities:

I > Br > Cl > S > F > O > N > 13C > 12C > 3H > 2H > 1H

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(R) and (S) Nomenclature

  • In case of ties, use the next atoms along the chain as tiebreakers.





CH(CH3)2 > CH2CH2Br > CH3CH2

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(R) and (S) Nomenclature

  • Treat double and triple bonds as if both atoms in the bond were duplicated or triplicated:

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(R) and (S) Nomenclature

  • Using a 3-D drawing or model, put the 4th priority group in back.

  • Look at the molecule along the bond between the asymmetric carbon and the 4th priority group.

  • Draw an arrow from the 1st priority group to the 2nd group to the 3rd group.

    • Clockwise arrow (R) configuration

    • Counterclockwise arrow (S) configuration

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(R) and (S) Nomenclature

Example: Identify the asymmetric carbon(s) in each of the following compounds and determine whether it has the (R) or (S) configuration.

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(R) and (S) Nomenclature

Example: Name the following compounds.

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(R) and (S) Nomenclature

  • When naming compounds containing multiple chiral atoms, you must give the configuration around each chiral atom:

    • position number and configuration of each chiral atom in numerical order, separated by commas, all in ( ) at the start of the compound name

(2S, 3S)-2-bromo-3-chlorobutane

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Depicting Structures with Asymmetric Carbons

Example: Draw a 3-dimensional formula for (R)-2-chloropentane.

Step 1:Identify the asymmetric carbon.


Step 2:Assign priorities to each group attached to the asymmetric carbon.





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Depicting Structures with Asymmetric Carbons

Step 3:Draw a “skeleton” with the asymmetric carbon in the center and the lowest priority group attached to the “dashed” wedge (i.e. pointing away from you).

Step 4:Place the highest priority group at the top.

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Depicting Structures with Asymmetric Carbons

Step 5:For (R) configuration, place the 2nd and 3rd priority groups around the asymmetric carbon in a clockwise direction.

Step 6:Double-check your structure to make sure that it has the right groups and the right configuration.

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Depicting Structures with Asymmetric Carbons

Example:The R-enantiomer of ibuprofen is not biologically active but is rapidly converted to the active (S) enantiomer by the body. Draw the structure of the R-enantiomer.

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Depicting Structures with Asymmetric Carbons

Example: Captopril, used to treat high blood pressure, has two asymmetric carbons, both with the S configuration. Draw its structure.